The present invention relates to a process for producing a preform for a metal matrix composite. More particularly, the present invention is directed to a process for producing a preform for use as a reinforcement in the formation of a metal matrix composite to strengthen the site of a material subject to severe thermal load, for example, a piston head of an internal combustion engine.
A ceramic whisker, for example, a whisker of silicon carbide or silicon nitride, has been put to practical use as a reinforcement for a member exposed to severe service environment, for example, an engine member, by virtue of its excellent mechanical strength, thermal stability and chemical stability. For example, in an internal combustion engine, the piston crown repeatedly undergoes a severe thermal load which brings about fusing damage or cracking. For this reason, Roger Anthony Day proposed to composite and strengthen the piston crown with a ceramic whisker, which has contributed to a significant improvement in the service life of the piston (see U.K. Patent No. 2106433). According to this patent, the compositing is locally conducted between the member reinforced with a ceramic whisker and the metal. The local compositing, however, is unfavorable because it is liable to bring about breaking due to the occurrence of cracking attributable to a difference in the thermal expansion between the materials at the boundary between the composited portion and the non-composited portion during the use of the piston, or the blistering of the matrix. For this reason, in order to avoid a rapid change in the thermal expansion behavior at the boundary between the composited portion and the non-composited portion, a proposal has been made of an inorganic-fiber-reinforced composite member having the so-called "functionally gradient structure". In this structure the volume fraction (Vf) of the reinforcing inorganic fiber in the non-composited portion is made smaller than that in the center of the composited portion and, at the same time, the volume fraction (Vf) is gradually increased with a gradient from the boundary between the non-composited portion and the composited portion towards the center of the composited portion (see Japanese patent application Kokai publication No. 60-190545). Further, a proposal has also been made of a ceramic-whisker-reinforced light alloy composite material wherein a buffer composite portion is continuously interposed between the composited portion and the non-composited portion (metal matrix portion) (see Japanese patent application Kokai publication No. 2-88730).
The feature of each of the Japanese patent application Kokai publications No. 60-190545 and No. 2-88730 resides in producing a preform having a functionally gradient structure from a reinforcing fiber, disposing it on a matrix metal at a site to be reinforced, and pressure casting the resultant matrix metal.
The above-described prior art method, however, has a drawback that the step of producing a preform having a functionally gradient structure is troublesome and unsuitable for practical use as a mass production technique.
Specifically, in the method disclosed in the Japanese patent application Kokai publication No. 60-190545, it is necessary to make a preform for the compositing from a single inorganic fiber molding wherein the density increases from the boundary between the composited portion and the non-composited portion towards the center of the composited portion, or to prepare a plurality of preforms different from each other in the density and disposing these preforms in combination so that the density increases from the boundary between the composited portion and the non-composited portion towards the center of the composited portion. Therefore, in this method, a troublesome and difficult working procedure is required in forming the preform.
In the method disclosed in the Japanese patent application Kokai publication No. 2-88730, the preform is produced by filtering a ceramic fiber having a diameter of 2 to 15 .mu.m to form a primary processed preform having a volume fraction (Vf) of 2 to 8 %, pouring a dispersion containing a ceramic whisker having a diameter of 2 .mu.m dispersed therein on one surface of the primary processed preform to again conduct the filtration treatment through the use of the primary processed preform as a filter medium, thereby impregnating the ceramic whisker into the primary processed preform structure so that the volume fraction (Vf) has a continuous gradient from 10 to 35%. Since, however, the filtration treatment is conducted in both the step of forming a primary processed preform and the step of forming a preform, the filtration treatment should be conducted twice. In particular, in the second filtration (the step of filtering a ceramic whisker), clogging occurs, which causes the working time to be prolonged, so that the manufacturing procedure becomes troublesome.